Apparatus and methods are known for making duplicate magnetic records using anhysteretic or thermoremanent transfer. Typically, a web on which the master recording has been made and a web onto which a mirror image of the master recording is to be transferred are caused to travel in intimate contact through a signal transfer zone where a decaying, alternating magnetic field is applied for anhysteretic recording or a beam of energy is applied for thermoremanent transfer. Inadequate contact or slippage between the master and slave introduces modulation and distortion of the magnetic signal being transferred. Such distortion affects signal quality and may introduce errors in digital data signals which are transferred to the slave. The problem of maintaining intimate contact between the master and slave webs is aggravated when the webs are driven at speeds in excess of 60 inches per second (1524 mm per second) and when the webs are as wide as 1.37 inches (35 mm). At such speeds the webs are likely to entrain boundary layers of air which tend to separate them, in spite of tension or pressure applied to the webs. The difficulty of obtaining the required intimate contact between wider master and slave webs in the signal transfer zone has limited use of contact duplication processes for such webs, particularly when highly reliable transfer is required.
In order to maintain intimate contact in the signal transfer zone it has been proposed to use pressure pads, pressure rollers or belts and air clamps. Various air or pneumatic clamp designs have been proposed, some including vacuum clamps. Where excess air pressure has been used to press the webs into contact, as much as five times atmospheric pressure has been used for narrower webs. The large number of proposals for providing intimate contact between the master and slave webs indicates that the problem of obtaining the desired intimate contact still awaits a satisfactory solution, particularly for wider webs.
The problem concerns removal of the entrained air in the boundary layers which attach themselves to the master and slave webs and are retained between the webs after they are brought into contact. This problem differs from the problem of air trapped between the convolutions of a web while it is being wound onto a reel. Such trapped air can change winding tension and prevent compact and uniform winding. In such applications, a vacuum has been used to eject air from between the incoming web and the preceding convolution on the reel. In such prior art apparatus, the differential pressure caused by the vacuum acts perpendicularly to the web and can cause jitter of the web, which is not a problem when a web is being wound onto a reel. But, such jitter can affect adversely the relative positions of master and slave webs and introduce distortion in a transferred magnetic signal.
The slave or copy web in some applications may be a photographic film. The magnetic recording medium on the slave web may be of the type shown in commonly assigned U.S. Pat. Nos. 3,782,947 and 4,302,523; in Research Disclosures Item 34390 of November 1992; and in commonly assigned published International Application WO91/11750. For photographic applications, it is desired to record, particularly along an edge or both edges of the film, magnetic signals which can be played back in a camera or photo finishing machine to set the camera for picture taking or the photo finishing machine for processing of an exposed film. The problem of contact duplication is particularly difficult when the slave web is photographic film, since damage must be prevented to the photosensitive layer or emulsion on the film. The master web may be of the type disclosed in commonly assigned, copending, allowed U.S. application Ser. No. 811,386 filed 20 Dec. 1991.